Attachment of gelatin-grafted polymer particles to pre-precipitated silver halide grains

Abstract: The invention describes silver halide packet emulsion grains or crystals (11) that are conventionally precipitated using peptizing gelatin (12) of a given isoelectric pH, surrounded by a layer of gelatin-grafted-polymer particles (16) wherein the grafted gelatin (12) has a different isoelectric pH than the peptizing gelatin (12). The gelatin-grafted-polymer particles (16) are optionally chemically bonded to the gelatin (12) surrounding the silver halide grains (11). Such packet emulsions can form the basis for a mixed-packet color photographic system.

Description: This invention relates to a photosensitive silver halide emulsion composition, a method of preparing said composition and to a mixed packet photosensitive photographic element. Photographic emulsions typically comprise silver halide particles dispersed in an aqueous medium. Traditionally, various types of gelatin have been used as a peptizer for the precipitation of photographic silver halide emulsions. This results in a layer of adsorbed gelatin surrounding each silver halide grain. The hydrated thickness of the gelatin layer may vary anywhere from 10 to 60 nm. Silver halide particles comprising silver halide grains each surrounded by a layer of peptizing gelatin are referred to herein as "silver halide-gelatin particles". Our copending European patent application EP-A-606 077 describes the precipitation of Ag-halide emulsions in the presence of gelatin-grafted-polymer particles comprising a photographically useful compound. By the method disclosed in this copending application one obtains polymer particles directly attached to the Ag-halide microcrystals. As elucidated therein, there are many advantages associated with having such polymer particles attached to silver halide grains in emulsion systems, including the preparation of mixed packet photographic systems. However, the method described in this patent application requires modification of known emulsion preparation processes to optimize the process for use with the gelatin-grafted-polymer particles. There is a need to improve delivery of photographically useful compounds to silver halide particles in a photographic emulsion without having to modify and/or optimize conventional emulsion forming processes. In particular, there is a need to improve delivery of photographically useful compounds to specifically sensitized silver halide particles specific to the spectral sensitivity of the silver halide particles in order to form mixed-packet color photographic systems. We have discovered that the advantages of associating gelatin-grafted-polymer particles with silver halide particles as set forth in our above-mentioned copending application can be achieved with preformed, pre-precipitated, conventional silver halide emulsions. This permits the use of silver halide emulsions prepared by conventional manufacturing techniques well known and/or optimized for a particular photographic element. One aspect of this invention comprises a photosensitive silver halide emulsion composition comprising in an aqueous medium: (a) silver halide-gelatin particles comprising silver halide grains, each surrounded by a layer of adsorbed gelatin wherein the gelatin has an isoelectric pH of P 1 ; and (b) gelatin-grafted-polymer particles wherein the gelatin has an isoelectric pH of P 2 which is different than P 1 ; wherein the gelatin-grafted-polymer particles are attached to the layer of gelatin surrounding the silver halide grains. The attachment of the gelatin-grafted-polymer particles to the silver halide particles may be physical or chemical. Another aspect of this invention comprises a method of preparing a photographic silver halide emulsion composition comprising: (i) mixing in an aqueous medium (a) silver halide-gelatin particles comprising silver halide grains, each surrounded by a layer of adsorbed gelatin, in which the gelatin has an isoelectric pH of P 1 ; and (b) gelatin-grafted-polymer particles in which the gelatin has an isoelectric pH of P 2 which is different than P 1 ; and (ii) adjusting the pH of the aqueous medium to a value that is between P 1 and P 2 , whereby gelatin-grafted-polymer particles are attached to the silver halide gelatin particles. The method can further comprise the step of cross linking the gelatin-grafted-polymer latex particles to the gelatin surrounding the silver halide grains using a gelatin hardener. Yet another aspect of this invention comprises a mixed-packet photosensitive photographic element comprising at least one of the following packet emulsion elements: silver halide particles sensitive to red light and comprising silver halide grains each surrounded with a layer gelatin wherein the gelatin has an isoelectric pH of P 1a and attached thereto gelatin-grafted-cyan dye-forming coupler particles wherein the gelatin has an isoelectric pH of P 2a which is different than P 1a , silver halide particles sensitive to green light and comprising silver halide grains each surrounded with a layer gelatin wherein the gelatin has an isoelectric pH of P 1b and attached thereto gelatin-grafted-magenta dye-forming coupler particles wherein the gelatin has an isoelectric pH of P 2b which is different than P 1b , silver halide particles sensitive to blue light and comprising silver halide grains each surrounded with a layer gelatin wherein the gelatin has an isoelectric pH of P 1c and attached thereto gelatin-grafted-yellow dye-forming coupler particles wherein the gelatin has an isoelectric pH of P 2c which is different than P 1c . In each packet element the gelatin of the two types of particles may be chemically bonded with a gelatin cross linking agent. The invention has numerous advantages over prior photographic products and processes for their production. The invention provides gelatin-grafted-polymer particles loaded with photographically useful compounds or gelatin-grafted-polymeric photographically useful compounds attached to the gelatin layer surrounding a conventionally pre-precipitated silver halide grains. These photographically useful compounds are in close association with the silver halide grains and therefore can readily react during photographic processing. The ability to mix different spectrally sensitized silver halide grains that are surrounded by dye forming coupler particles complementary to the spectral sensitization of the silver halide particles allows mixing in one silver halide layer of a photographic element, packets of magenta, cyan and yellow dyes with development only of the coupler that is bound to the gelatin layer surrounding a particular sensitized silver halide grain. Additionally, compositions comprising soft polymer particles tend to be less pressure sensitive than conventional silver halide emulsion compositions. FIG. 1a illustrates a silver halide-gelatin particle which comprises a silver halide grain precipitated in an aqueous gelatin medium. FIG. 1b illustrates a gelatin-grafted-polymer particle. FIG. 1c illustrates the pH dependence of charge of standard lime processed ossein gelatin and that of standard lime processed ossein gelatin-grafted-polymer particles. FIG. 1d illustrates gelatin-grafted-polymer particles attached to a pre-precipitated silver halide-gelatin emulsion particle. FIG. 2 is a conceptual depiction of a three color mixed-packet color photographic element achieved by the method of this invention. FIG. 3 is a shadowed electron photomicrograph of latex of Example-1. FIG. 4 illustrates the pH-dependence of the hydrodynamic size of the polymer latex of Example-1, as measured by photon correlation spectroscopy. FIG. 5 illustrates the pH-dependence of the hydrodynamic size of the gelatin-grafted-polymer latex of Example-2, as measured by photon correlation spectroscopy. FIG. 6a is a scanning electron photomicrograph of emulsion of Example-6, precipitated with lime processed ossein gelatin and Fig. 6b is a scanning electron photomicrograph of Example-8, where gelatin-grafted-polymer latex [35% Gel] of Example-2 are attached to the AgCl grains of the emulsion of Example-6. Please note that the magnification of FIG. 6a is half that of FIG. 6b. FIG. 7a is a scanning electron photomicrograph of tabular grain emulsion of Example-7 precipitated with lime processed ossein gelatin and FIG. 7b is a scanning photomicrograph of tabular grain emulsion of Example-9, where gelatin-grafted-polymer latex particles [30% phthalated gelatin] of Example-5, were attached to the surface of the AgBr(I 3%) gelatin surrounded grains of the emulsion of Example-7. FIG. 8 is an enlarged view of an emulsion grain of Example-9. This invention provides gelatin-grafted-polymer particles attached to conventionally preprecipitated silver halide emulsion grains, each with a layer of its own peptization gelatin around the silver halide grain. A silver halide grain together with its layer of peptizing gelatin is referred to herein as a "silver halide-gelatin particle". Fig. 1a illustrates a conventionally precipitated silver halide particle 10 comprising a silver halide tabular grain 11 and a surrounding gelatin layer 12. It is to be understood that the term "conventionally" merely states that the silver halide grains are prepared in an aqueous medium containing peptizing gelatin that adheres to the grains. Such processes are conventional. It is recognized that improvements of the basic process may be made from time to time. It is contemplated that any silver halide grains produced by precipitation in an aqueous gelatin-containing medium are suitable for use in this invention, even if the details of the precipitation process are developed hereafter. Various types of methods used in the preparation of photographic silver halide emulsions have been described in detail in T. H. James, "The Theory of the Photographic Process," 4th Edition, New York (1977) (hereinafter "James") and U.S. Patent Nos. 4,334,012 and 4,879,208. The emulsion may be a AgCl, AgBr, AgI, AgCl(Br), AgCl(I), AgClBr(I), or AgBr(I) emulsion. Preferred are silver halide grains comprising silver chloride, silver iodobromide, or silver chlorobromide. The silver halide grains preferably have a single dimension ranging between about 10 nm to about 10,000 nm. The weight of gelatin used for precipitation of silver halide-gelatin particles for use in this invention depends on the crystal morphology or shape of the silver halide grains to be prepared and their sizes. It may range from about 2 grams of gelatin to about 200 grams of gelatin per mole of the silver halide emulsion prepared. The amount is determined by the size of the emulsion grains, such that after the emulsion is formed substantially all the gelatin is bound to the silver halide grain surface, as discussed more fully below. The emulsion particles may be cubic, octahedral, rounded octahedral, polymorphic, tabular or thin tabular emulsion grains. Preferred are silver halide grains having a cubic, octahedral, or tabular crystal structure. Such silver halide grains may be regular untwinned, regular twinned, or irregular twinned with cubic or octahedral faces. The gelatin starting material may be a regular lime processed or acid processed ossein gelatin or various derivatized gelatins as described in James and U.S. Patent No. 5,026,632 . Gelatins such as phthalated, acetylated, or alkylated gelatins, such as succinated gelatin, are particularly useful in some embodiments of this invention. Variation of the types of gelatin provides variations in the isoelectric pH of the formed particles. This variation in the isoelectric pH provides the basis of particle attachment, as discussed in more detail below. The gelatin adsorbed on the silver halide grains has an isoelectric pH of P 1 . Generally, the amount of gelatin surrounding each grain should be about 10 mg per sq meter of the surface of the emulsion grains. This consideration is similar to that provided for the gelatin-grafting-polymer particles, as discussed more fully below. Fig. 1b illustrates a gelatin-grafted-polymer particle 16 comprising a polymer core 17 and a surrounding gelatin layer 18. The preparation of gelatin-grafted-polymer particles has been extensively described earlier, for example, in U.S. Patent Nos. 4,920,004, 4,855,219, 5,066,572, 5,055,379, and 5,026,632. Polymers useful in the preparation of gelatin-grafted-polymer particles are any polymers capable of covalently bonding with gelatin, either directly or with the aid of a grafting agent. Preferred polymers that covalently bond directly with gelatin are homopolymers and copolymers of monomers containing active halogen atoms, isocyanates, epoxides, monomers containing aldehyde groups, and monomers containing chloroethylsulfone groups or vinyl sulfone groups. Preferred polymers that are capable of bonding with gelatin through the use of a cross linking agent include carboxylic acids, amine-containing monomers, and active methylene group-containing monomers. Generally, the polymer particles are formed by emulsion polymerization, suspension polymerization, or limited coalescence to form a latex. The polymer particles in the latex generally have a diameter of about 10 to about 10 6 nm. As mentioned above, the gelatin is then monomolecularly bonded to the surface of the polymer particles of the latex by direct chemical reaction or by the use of a chemical grafting agent. A gelatin grafting agent is a chemical compound that will allow bond formation between gelatin and a chemical moiety on the surface of the polymer particle. Typical of such chemical grafting agents suitable for the invention are carbamoylonium compounds, dication ether compounds, and carbodiimide compounds, for example the compounds disclosed in above-mentioned U.S. Patent No. 5,066,572. Of particular importance to this invention are the gelatin-grafted-polymer particles that have been prepared such that there is substantially no excess gelatin remaining in solution of the gelatin-grafted-polymer latex system. In other words, the gelatin-grafted-polymer samples that are useful for this invention have substantially all the gelatin molecules bound to the polymer particle surface. Therefore, the amount of gelatin to be used depends upon the specific surface area (S) of the latex particles. The specific surface area of polymer particles depends upon the mean particle diameter of the particle (D). S is given by S = 6ρ/D (1) where ρ is the density of the polymer particle. The saturation adsorption of gelatin depends upon the pH and ionic strength of the solution. However, as a general rule the saturation adsorption of about 10 mg/sq meter of surface is a reasonable estimate. See J. Phys. Chem. 63, 3009 (1964) by Curme et al. and U.S. Patent No. 5,091,296. The gelatin-grafted-polymer particles of this invention are those that have been prepared at gelatin coverages that are less than about 10 mg of gelatin per sq meter of the polymer particle surface and preferably below about 8 mg of gelatin per sq meter of the polymer particle surface. The gelatin starting material used to prepare the gelatin-grafted-polymer particles may be a regular lime processed or acid processed ossein gelatin or various derivatized gelatins as described in James and U.S. Patent No. 5,026,632. Gelatins such as phthalated, acetylated, alkylated, or succinated gelatin, may be particularly useful in some embodiments of this invention. Variation of the types of gelatin provides variations in the isoelectric pH of the formed particles. The gelatin in the gelatin-grafted-polymer particles has an isoelectric pH of P 2 , which is different from P 1 , the isoelectric pH of the gelatin adsorbed on the pre-precipitated silver halide grains. The difference between P 1 and P 2 should be at least about one unit of pH value, preferably at least about 1.5 units, and more preferably about 2.0 units. P 2 generally differs from the isoelectric pH of the gelatin starting material, as illustrated in Fig. 1c. In Fig 1c, the line P represents the pH dependence of charge of standard lime processed ossein gelatin and the line Q represents that of standard lime processed ossein gelatin-grafted-polymer particles. In general, the gelatin starting material may be the same as the gelatin starting material used for preparing the silver halide-gelatin particles or it may be a different gelatin, providing that the gelatin when attached to the silver halide grains has a different isoelectric pH than when grafted onto the polymer particles. This is due to the reaction of some of the amine group in the gelatin molecule during the grafting reaction. In accordance with this invention, gelatin-grafted-polymer particles are attached to the gelatin surrounding the pre-precipitated silver halide grains. The resulting composite particle is shown in Fig. 1d. In Fig. 1d, gelatin-grafted-polymer particles 16, comprising polymer core 17 and gelatin 18, are attached to silver halide particle 10, comprising a silver halide grain 11 and a layer of absorbed gelatin 12. The gelatin-grafted-polymer particles are attached to the silver halide-gelatin particles by mixing the two types of particles in an aqueous medium and adjusting the pH of the medium by adding base or acid, as appropriate, to a pH value between the isoelectric pH values of the layers of gelatin surrounding the two different types of particles, that is between P 1 and P 2 . Any base or acid can be used to adjust the pH. Preferred acids and bases include, for example, sulfuric acid, nitric acid and sodium hydroxide. The process of physical attachment of the gelatin-grafted-polymer particles involves the dissimilarity of the net charge at a given pH between the gelatin bonded to the surface of the gelatin-grafted-polymer particles and the gelatin adsorbed on the surface of the silver halide particles, as depicted in Fig. 1c. If the pH of the medium is between P 1 and P 2 , the charge on the outer gelatin layers of the two types of particles are opposite and the gelatin-grafted-polymer particles will be attached to the gelatin coated silver halide grains. This opposite charge interaction forms the basis for the physical attachment (prior to chemical bonding) of the gelatin-grafted-polymer particles to the silver halide-gelatin particles. The gelatin-grafted-polymer particles used in an amount sufficient to surround substantially the surface of the individual silver halide-gelatin particles. The process described above results in composite particles in which the gelatin layer of the pre-precipitated silver halide particles is physically attached to the gelatin of the gelatin-grafted-polymer particles. The gelatin of the component particles can be further chemically attached by using a gelatin cross linking agent. As there is little, if any, unbound gelatin in solution, the process will cause the gelatin-grafted-polymer particles to be chemically bonded to the outer gelatin layer of the silver halide particle. The cross linking agent used is preferably a gelatin hardener such as bisvinylsulfonylmethane ether, bisvinylsulfonylmethane, carbamoylonium compounds, dication ether compounds, carbodiimide compounds. Preferred cross linking agents are disclosed in U.S. Patent No. 5,026,632. Generally the invention is accomplished by the use of gelatin-grafted-polymer particles that are preferably loaded or imbibed with photographically useful compounds, such as couplers. The photographically useful compounds can also be incorporated in the core polymer of the gelatin-grafted-polymer particles, by the use of a polymeric photographically useful compound as the core polymeric particle. The chemical compositions of the core polymer particles have been described extensively in U.S. Patents Nos. 4,920,004, 4,885,219, 5,066,572, 5,055,379, and 5,026, 632. The core polymer particle of the gelatin-grafted-polymer particles utilized in this invention can be loaded with one or a combination of the following types of photographic agents by the methods described in U.S. Patent Nos. 4,199,363 and 5,091,296: a. Filter Dyes, b. Development Inhibitor Release Couplers, c. Development Inhibitor Anchimeric Release Couplers, d. Dye-Forming Couplers, e. Nucleators, f. Development accelerators, g. Ultraviolet Radiation Absorbing Compounds, h. Sensitizing Dyes, i. Development Inhibitors, j. Antifoggants, k. Bleach Accelerators. Attachment of photographic agents to silver halide-gelatin particle surfaces in many cases can improve the colloidal stability of the photographic emulsion as the thickness of the protective layer around the silver halide grain is now much greater than a layer of gelatin. The chemical compositions of the core polymeric photographic agent particles, useful for this invention, have been described extensively in U.S. Patents Nos. 4,855,219, 5,066,572, 5,055,379 4,877,720, 4,464,462, and 4,080,211. Typical polymeric core photographic agent particles suitable for this invention are as follows: a. Polymeric Filter Dye Particles, b. Polymeric Development Inhibitor Release Coupler Particles, c. Polymeric Development Inhibitor Anchimeric Release Coupler Particles, d. Polymeric Dye-Forming Coupler Particles, e. Polymeric Ultraviolet Radiation Absorbing Compound Particles, f. Polymeric Development Accelerator Particles, g. Polymeric Developer Particles, h. Polymeric Sensitizing Dye Particles, i. Polymeric Development Inhibitors, j. Polymeric Antifoggants, k. Polymeric Bleach Accelerators. Attachment of photographic agents to the preformed, pre-precipitated silver halide emulsion particles can improve the photographic performance of photographic products, in many cases. It is known that the incorporation of gelatin-grafted-soft polymer particles in photographic layers with silver halide emulsions can vastly improve the pressure sensitivity of photographic film products, without hindering developability of the photographic film, for example, see U.S. Patents Nos. 4,855,219, 5,066,572, 5,055,379, and 5,026,632. As described in these patents, the polymer core of the gelatin-grafted-soft polymer particles is a polymer that is soft and deformable, preferably with a glass transition temperature of less than 25 degrees C and capable of being covalently bonded to gelatin, either directly of with the aid of a cross linking agent. Suitable materials are those polymer latex particles described in the above mentioned patents. A layer of soft gelatin-grafted-polymer particles attached to the gelatin layer surrounding pre-precipitated silver halide particles surface is believed to provide enhanced and improved pressure sensitivity of photographic elements, particularly those prepared from highly pressure sensitive thin tabular grain emulsions. In other embodiments, this invention provides a mixed-packet color photographic coating as pictorially indicated in Fig. 2. In Fig. 2, support 20 has on a surface thereof a layer 21 comprising composite particles 22a, 22b and 22c, each comprising gelatin-grafted-polymer particles 16a, 16b and 16c which contain cyan-, magenta- and yellow-dye forming couplers, respectively, attached to the gelatin layer of silver halide-gelatin particles 10a, 10b and 10c which have been sensitized to red, green and blue light respectively. Thus the mixed packet photographic element is composed of red, blue, and green sensitized silver halide emulsions mixed in a single layer with the red emulsion associated with attached cyan dye-forming coupler, the green emulsion associated with magenta dye-forming coupler, and the blue emulsion associated with yellow dye-forming coupler. A dispersion of oxidized developer scavenger may be interspersed among the packet emulsions to prevent color contamination between component particles. The composite particles are separately prepared as discussed above for each color using (a) red sensitive silver halide grains having on the surface thereof adsorbed gelatin having an isoelectric pH of P 1a and gelatin-grafted-polymer particles comprising a cyan dye forming coupler, in which particles the gelatin has an isoelectric pH of P 2a which is different than P 1a ; (b) green sensitive silver halide grains having on the surface thereof adsorbed gelatin having an isoelectric pH of P 1b and gelatin-grafted-polymer particles comprising a magenta dye forming coupler in which particles the gelatin has an isoelectric pH of P 2b which is different than P 1b ; and blue sensitive silver halide grains having on the surface thereof adsorbed gelatin having an isoelectric pH of P 1c and gelatin-grafted-polymer particles comprising a yellow dye forming coupler in which particles the gelatin has an isoelectric pH of P 2c which is different than P 1c . The silver halide packet emulsion prepared by the method of this invention, allows the attachment or adsorption of gelatin-grafted-polymeric dye-forming coupler particles or gelatin-grafted-dye-forming coupler loaded polymer particles to the silver halide-gelatin particles. Therefore, the dye-forming coupler by the method of this invention is intimately associated with the silver halide particles. Preparation of red sensitized silver halide packet emulsions using gelatin-grafted-cyan coupler particles, green sensitized silver halide packet emulsions using gelatin-grafted-magenta coupler particles, and blue sensitized silver halide packet emulsions using gelatin-grafted-yellow coupler particles and coating them in a single layer as shown in Fig. 2 can provide a high resolution mixed-packet color photographic system. The resolution would be high as the packet emulsion is formed by a single silver halide grain surrounded by the coupler(-containing) particles. These preformed silver halide-gelatin emulsion particles having gelatin-grafted-polymers adhered to them may be utilized in conventional photographic materials as well as in the mixed-packet photographic elements. In other embodiments of the invention the silver halide grains may be sensitized to infrared or ultraviolet light. The support can be any suitable support used with photographic elements. Typical supports include polymeric films, paper (including polymer-coated paper) and glass. Details regarding supports and other layers of the photographic elements of this invention are contained in Research Disclosure, December 1978, Item 17643, referred to above. The support can be coated with a magnetic recording layer as discussed in Research Disclosure 34390 of November 1992. As described above this invention provides photographic agents such as filter dyes, development inhibitor release couplers, development inhibitor anchimeric release couplers, dye-forming couplers, nucleators, ultraviolet radiation absorbing materials, development accelerators, developers, sensitizing dyes, and various photographic agents close to the silver halide grain surface by incorporating or loading such agents into polymer particles then grafting gelatin to the particles and attaching the resulting gelatin-grafted-polymer particles to silver halide-gelatin preprecipitated particles. This results in the photographic agent being in close proximity with the silver halide grain surface. The following examples are intended to be illustrative and not exhaustive of the invention. Parts and percentages are by weight unless otherwise mentioned. Example 1: Preparation of Poly(Styrene-co Butylacrylate-co-Ethylene Glycol Dimethacrylate-co-Methacrylic Acid) Latex - Weight Ratio (37/37/2/24) The latex was prepared by standard emulsion polymerization procedure as follows. A 5 L 3-neck round-bottom flask fitted with a condenser, an air stirrer and a supply for nitrogen under low blanketing pressure was charged with 4 L of nitrogen purged distilled water. The flask was placed in a constant temperature bath (CTB) at 60°C. After temperature equilibration 0.4 g of sodium dodecylsulfate surfactant was added to the reaction flask and a mixture of the following monomers: Styrene 148 g Butylacrylate 148 g Methacrylic Acid 96 g Ethylene Glycol Dimethacrylate 8 g TOTAL 400 g To the formed emulsion was added 8 g of (NH 4 ) 2 S 2 O 8 and 4 g of Na 2 S 2 O 5 . The polymerization reaction was carried out for 18 h at 60°C. The latex was dialyzed against distilled water for 24 h in a continuous dialysis set up. The dialyzed latex had a solids contrast of 8.4%. The particle size of the latex was measured by photon correlation spectroscopy to be 80 nm. Figure 3 shows a representative shadowed electron photomicrograph of the latex particles. They appear to be indeed around 80 nm. This latex is designated as Latex (Example-I). Example 2: Preparation of Gel-g-Latex (Example-I) [35% Gel-(A)] In order to use gelatin-grafted-polymer particles to attach to pre-precipitate emulsions, it is necessary to prepare gel-g-latex particles with no excess gelatin remaining in solution such that there is very little or no free gelatin to attach to the gel-silver halide particles. Therefore, all gelatin-grafting procedures in this work were carried out with less gelatin than that necessary to completely cover the surface. Gelatin adsorption on Ag-halide surfaces has been extensively studied by Curme et al., referenced above. As expected for polypeptides that contain -COOH and-NH 2 groups, this adsorption excess is highly dependent on pH and ionic strength. An estimate for use in synthetic work is about 10 mg of gelatin adsorbed at saturation per sq meter of surface. The latex of Example 1 with a diameter of 80 nm has a surface area of 75 m 2 /g. Therefore for 75% coverage of surface, we need about (75 m 2 /g x 0.75 x 0.01 g/m 2 ) = 0.56 g of gelatin per gram of the dry latex polymer. In other words, in the dry gel-g-latex polymer there will be [(0.56/1.56) x 100] = 35% gelatin. Gelatin used in this example is standard lime processed ossein gelatin designated as gelatin (A). Based upon the above analysis, gelatin grafting to the latex of Example 1 was carried out as follows. 1190 g of the latex of Example 1 containing 100 g of dry polymer was adjusted to pH = 8.0 using 20% NaOH solution and heated to 60°C in 3-neck round-bottom flask. 52.9 g of deionized lime processed ossein gelatin (12.5% moisture) was dissolved in 500 g of water and heated to 60°C, and the pH was adjusted to 8.0 using 20% NaOH. 3.5 g of the gelatin grafting agent (I) (based upon 0.2 moles of (I) per mole of surface methacrylic acid, taken to be 5% of the polymer particle by weight) was added to the latex at 60°C and stirred for 15 min. Then the gelatin solution at 60°C was added to the latex dispersion and reacted for another 15 min. The gel-g-latex material was called gel-g-latex (Example I) [35% Gel-A] and had a solid constant of 9.0%. However, samples for all photographic testing were dialyzed at 40°C continuously against distilled water to remove the fragments generated in the grafting reaction. See reaction scheme. The chemistry of gelatin-grafting to carboxylated particles is generally assumed to proceed according to any of the following pathways. Example 3: Physical Characteristics of Gel-g-Latex of Example 2 High carboxylic acid containing latexes are known to swell with increase in pH due to the ionization of the carboxylic acid groups. Figure 4 shows that in the case of latex of Example-1 swelling taking place around pH = 8.0. This is greater than the pKa of carboxylic acid groups, as the van der Waals' attraction between the hydrophobic comonomers as butylacrylates and styrene resist swelling. The full charging of carboxyl groups must take place before the van der Waals' forces can be overcome. At pH 11 the 80 nm particles are capable of swelling to about 120 nm, which corresponds to about 3.4 times the volume of the unswollen particles. It is seen in U.S. Patent No. 4,920,004, however, that at swamping ionic strengths (1 M KNO 3 ) the swelling of the latex at high pH does not take place, indicating that the observed swelling is induced by the repulsion of the ionized latex particle. Figure 5 shows a similar plot of the pH dependence of the hydrodynamic diameter of gel-g-latex of Example-2 at low and swamping electrolyte concentrations. It has been shown in U.S. Patent No. 4,920,004 that gelatin adsorbed Ag halide particles show a pH dependence of the hydrodynamic size due to the ionization of the -COOH and -NH 2 groups of gelatin. Below the isoelectric pH (IEP) of gelatin, the amine groups are charged leading to expansion of the adsorbed layer and above the IEP, the -COOH groups are ionized again leading to the expansion of the adsorbed layer of gelatin. The IEP is characterized by the smallest hydrodynamic size corresponding to its most compact size in the uncharged form. In Fig. 5 is seen that the minimum of the hydrodynamic size occurred around pH = 4.0 for gel-g-latex of Example-2, indicating that under low ionic strength conditions the IEP of gelatin around the gel-g latex particle is 4.8. However, ungrafted line processed ossein gelatin has an IEP of 4.8. This is because during grafting the -NH 2 groups are used for grafting to the particles, and hence a loss of net positive charge. As indicated earlier, this phenomenon is very useful for attachment and packet formation with gelatin-grafted-polymer particles. It is interesting to note that the swelling of the inner core particle containing methacrylic acid above pH 8 can be observed over the gelatin swelling in Fig. 5. It is also seen in Fig. 5 that under swamping electrolyte conditions, the gel shell thickness is the same as that at the IEP. This also attests to the fact that observed particle expansion is due to ionization charging of the gel-g-latex particles. The gel-g-latex below pH = 7 with 1 M KNO 3 showed flocculation. This could be due to the shift of the IEP of gelatin to larger values at high ionic strengths, as observed by Cohen et al., Adv. Chem. Ser. 45, 198 (1973), in association with the desolvation of the bound gel shell at such high ionic strength. Table I shows a list of the isoelectric pH values of various gelatin and gel-g-latexes. ISOELECTRIC PH VALUES OF VARIOUS GELATINS AND GEL-G-LATEXES Material Isoelectric pH Comments Standard lime processed ossein gelatin (A) 4.8 Cohen et al. Gelatin (A) phthalated (B) 4.1 Cohen et al. Gel (A)-g-latex 4.0 This work Phthalated gel (B)-g-latex ∼3.3 Estimate Example 4: Preparation of Poly(styrene-co-butylacrylate-co-methacrylic acid) Latex - Weight Ratio (37/37/24) Preparation of the latex of Example-4 was carried out according to procedures described earlier in Example-1, except the amounts of monomers, initiators, and surfactant used were as follows: Styrene 152.0 g Butylacrylate 152.0 g Methacrylic Acid 96.0 g K 2 S 2 O 8 2.0 g K 2 S 2 O 5 1.0 g Sodium dodecyl sulfate 0.4 g Reaction was carried out at 60°C for 20 hrs. The resultant latex had a solid content of 8.3% and a PCS particle diameter of 95 nm. The calculated specific surface area of the latex was 63 m 2 /g. Example 5: Preparation of Gel-g-Latex (Example 4) [30% Phthalated Gelatin (B)] Gel-g-latex of Example-4 (30% phthalated gelatin (B)] was prepared much the same manner as before (Example-3). 4.11 kg of the latex (= 341 g of polymer) of Example-4 was heated to 60°C and adjusted to a pH of 8.0. 11.9 g of grafting agent (I) (0.2 mole per mole of surface methacrylic acid, assumed 5% as before) was added to the latex as a 10% aqueous solution and allowed to react at 60°C for 15 min. 145 g of phthalated gelatin B was dissolved in 1640 g of distilled water at 60°C and pH of 8.0. After 15 min of reaction of the latex with compound (I), the gel solution was added to the latex and reacted for another 15 min at 60°C. The amount of gelatin used was designed to cover about 75% of the latex surface with no gelatin left in solution as discussed before. The resultant gel-g-latex had a solids content of 8.4%. The physical characteristics of the latexes and gel-g-latexes of this invention are given in Table II. CHARACTERISTICS OF THE LATEX AND GEL-G-LATEX PARTICLES Latex Unswollen Particle Dia. of Latex in nm by PCS Surface Area of Latex in m 2 /g Solids of Latex in % Gel-g-Latex % of Latex Surface Covered by Grated Gelatin Solids of Gel-g-Latex in % of Example 1 80 75 8.4 of Example 2 75 9.0 of Example 4 95 63 8.3 of Example 5 75 8.4 Example 6: Preparation of Cubic AgCl Emulsion Make Kettle Rousselot lime processed ossein 10.8 g Nalco antifoam 0.7 ml Distilled water 2989.2 g Temperature 60 °C pH 5.05 Control set point pAg = 7.55 Silver Solution AgNO 3 0.1 M Salt Solution NaCl 0.4 M It is to be especially noted that this is a very low gel emulsion. Just enough gel was added to keep the emulsion peptized with virtually no gel left in solution. The kettle contents were melted at 45°C with stirring and then adjusted to pH = 5.05 at 40°C. The kettle temperature and pAg (= -log[Ag + ]) was set to control point of 7.55 with 5 M NaCl. Stirring rate was increased from 2500 to 4000 rpm. Solutions of 0.4 M NaCl and 0.1 M AgNO 3 were added by a double-jet precipitation method with an accelerated flow profile from 22 ml/min to 115 ml/min in 13.25 min. The flow rate was held constant at 115 ml/min for the remainder of the make, while maintaining the pAg at 7.55 by means of a Honeywell controller. The total run time was 39.9 min. After precipitation of the emulsion, the pH of the emulsion was lowered to 3.80 with 4.0 M HNO 3 . The emulsion was allowed to settle. An electron photomicrograph of the emulsion crystals are shown in Fig. 6a. In such a low gel preparation it is noted that some grain shapes are a bit irregular from cubes. EGA (electrolytic grain size analysis) indicated a number average cubic grain edge length of 480 nm. Example 7: Preparation of Tabular Grain AgBr(I 3%) Emulsion Make Kettle Oxidized lime processed ossein deionized gelatin 10.5 g Nalco antifoam 0.7 ml Deionized water 3961 g pH adjusted to 1.85 Initial temperature 35 °C Growth temperature 60 °C Initial set point pAg = 9.63 Control set point pAg = 8.94 Silver Solution AgNO 3 1.0 M Salt Solution NaBr 1.0 M Auxiliary Salt Solution (Tandem with Ag) KI 0.03 M The preparation was a triple jet make with an auxiliary salt solution of KI, whose flow was maintained in tandem with the silver flow. The Ag and the salt solutions were added to the kettle at rates of 53 and 56 ml/min, respectively, without controlling the pAg, in order to form nuclei under a twinning environment. Following nucleation for 30 sec, the pumps were stopped and the temperature was ramped to 60°C over a period of 15 min. The nuclei were held at 60°C for 3 min and then 1 litre of a solution containing 133.4 g of oxidized gelatin and 5.49 g of NaBr was dumped into the kettle. The pAg after the dump was 8.94. The pH was adjusted to 6.00 and then the Ag and the salt solutions were added to the kettle while controlling both the temperature and the pAg at the set points for a period of 63.5 min. The initial flow rate was 10 ml/min, ramped to 117 ml/min. The temperature was brought down to 40°C after the make, and it was washed as described in Example-3 of U.S. Patent No, 2,614,929. The final gelatin concentration was made up to 40 g per mole of silver halide. 1.0 g of 4-chloro-3,5-xylenol was added as a preservative. Image analysis of this emulsion gave an equivalent circular grain diameter of 1200 nm and coated reflection measurement provided an average grain thickness of 45 nm. Figure 7a shows an SEM picture of the grains of this emulsion. Example 8: Attachment of Gel-g-Latex [35% Gel] of Example 2 Onto the Surface of Gel Precipitated Cubic AgCl Emulsion Grains of Example 6 50 g of emulsion of Example-6 (0.036 mole/L) was allowed to stand at 40°C. The supernatant was decanted off and replaced with an equal volume of deionized water. This mixture was then heated to 40°C and 5 g of gel-g-latex [35% gel] of Example-2 was added to the emulsion. The pH was lowered to 3.6 and the mixture was allowed to stand. The supernatant was decanted and replaced with deionized water. This procedure was repeated twice more. The last time the emulsion was left in the concentrated form. The material was coated on a scanning electron microscope (SEM) stage, evaporation coated with gold/palladium for enhanced contrast. The SEM picture is shown in Fig. 6b. It is clearly seen that the gel-g-latex particles are attached to the surface of the AgCl crystals, with very few unattached gel-g-latex particles in the field. Even though this emulsion sample was not coated, it is expected that such emulsion grains would be photographically active, as the emulsion grains were prepared by normal and known gelatin precipitation procedures. Example 9: Attachment of Gel-g-Latex [30% Gel] of Example 5 Onto the Surface of Gel Precipitated Tabular AgBr(I 3%) Emulsion Grains of Example 7 0.05 g of sensitizing dye (compound II) was dissolved in 25 ml of methanol and was added to 0.05 moles of emulsion of Example-7 at 40°C. This mixture was heated from 40°C to 60°C in 12 min, held for 15 min at 60°C and then chilled down to 40°C. 60 g of gel-g-latex of Example-5 was added at 40°C, followed by the dropwise addition of 3.3 ml of an 1.8% of bis(vinylsulfonylmethane) to the emulsion with stirring. It was held at 40°C with stirring for 6 hrs. The emulsion was then chill set and stored at 4°C. Figure 7b shows SEM pictures of the emulsion grains after gold/palladium coating. It shows definite attachment of the gel-g-latexes to the Ag halide grains. Since the emulsion was not isowashed, the unattached grains were not removed and are also seen along with the gel-g-latex attached emulsion grains. The experiments show the use of a gelatin hardener to attach the gel-g-latex particles to the preformed Ag halide grain surface, rather than by charge interaction by lowering of pH. Figure 8 shows an enlarged view of the gel-g-latex attached emulsion grains of Fig. 7b. These grains were not coated and tested for photographic sensitivity, as such gelatin precipitated conventional grains are well known to be photographically active, and the material of this example is expected to be photographically active.

Claims: A photosensitive silver halide emulsion composition comprising in an aqueous medium: (a) silver halide-gelatin particles comprising silver halide grains, each surrounded by a layer of adsorbed peptizing gelatin wherein the peptizing gelatin has an isoelectric pH of P 1 ; and (b) gelatin-grafted-polymer particles wherein the grafted gelatin has an isoelectric pH of P 2 which is different than P 1 ; wherein the gelatin-grafted-polymer particles are attached to the layer of peptizing gelatin surrounding the silver halide grains. The composition of claim 1 wherein said gel-grafted polymer particles comprise a photographic agent selected from at least one member of the group consisting of: filter dyes, development inhibitor release couplers, development inhibitor anchimeric release couplers, dye-forming couplers, nucleators, accelerators for photographic development, ultraviolet radiation absorbing compounds, sensitizing dyes, development inhibitors, antifoggants, and bleach accelerators. The composition of claim 1 wherein said gel-grafted polymer particles comprise grafted gelatin and a polymer selected from at least one member of the group consisting of: polymeric filter dye, polymeric development inhibitor release coupler, polymeric development inhibitor anchimeric release coupler, polymeric dye-forming coupler, polymeric ultraviolet radiation absorbing compound, polymeric development accelerator, polymeric developer, polymeric sensitizing dye, polymeric development inhibitors, polymeric antifoggants, and polymeric bleach accelerators. The composition of claim 1 wherein at least one of said peptizing and grafted gelatins comprise a gelatin selected from the group consisting of: acid processed ossein gelatin, lime processed ossein gelatin, phthalated gelatin, acetylated gelatin, and succinated gelatin, and wherein the peptizing gelatin is different from the grafted gelatin. A method of preparing a photographic silver halide emulsion composition comprising: (i) mixing in an aqueous medium (a) silver halide-gelatin particles comprising silver halide grains, each surrounded by a layer of adsorbed peptizing gelatin in which the peptizing gelatin has an isoelectric pH of P 1 ; and (b) gelatin-grafted-polymer particles in which the grafted gelatin has an isoelectric pH of P 2 which is different than P 1 ; and (ii) adjusting the pH of the aqueous medium to a value that is between P 1 and P 2 , whereby gelatin-grafted-polymer particles are attached to the silver halide gelatin particles. The method of claim 6 wherein the difference between P 1 and P 2 is at least one unit of pH value. A mixed-packet photosensitive photographic element comprising at least one of the following packet emulsion elements: silver halide particles sensitive to red light and comprising silver halide grains each surrounded with a layer of peptizing gelatin wherein the peptizing gelatin has an isoelectric pH of P 1a and attached thereto gelatin-grafted-cyan dye-forming coupler particles wherein the grafted gelatin has an isoelectric pH of P 2a which is different than P 1a , silver halide particles sensitive to green light and comprising silver halide grains each surrounded with a layer of peptizing gelatin wherein the peptizing gelatin has an isoelectric pH of P 1b and attached thereto gelatin-grafted-magenta dye-forming coupler particles wherein the grafted gelatin has an isoelectric pH of P 2b which is different than P 1b , silver halide particles sensitive to blue light and comprising silver halide grains each surrounded with a layer of peptizing gelatin wherein the peptizing gelatin has an isoelectric pH of P 1c and attached thereto gelatin-grafted-yellow dye-forming coupler particles wherein the grafted gelatin has an isoelectric pH of P 2c which is different than P 1c . The element of claim 7 wherein (a) said gel-grafted polymer particles comprise a photographic agent selected from at least one member of the group consisting of: filter dyes, development inhibitor release couplers, development inhibitor anchimeric release couplers, dye-forming couplers, nucleators, accelerators for photographic development, ultraviolet radiation absorbing compounds, sensitizing dyes, development inhibitors, antifoggants, and bleach accelerators; or (b) said gel-grafted polymer particles comprise grafted gelatin and a polymer selected from at least one member of the group consisting of: polymeric filter dye, polymeric development inhibitor release coupler, polymeric development inhibitor anchimeric release coupler, polymeric dye-forming coupler, polymeric ultraviolet radiation absorbing compound, polymeric development accelerator, polymeric developer, polymeric sensitizing dye, polymeric development inhibitors, polymeric antifoggants, and polymeric bleach accelerators. The element of claim 7 wherein the peptizing gelatin of the silver halide gelatin particles and the grafted gelatin of the gelatin-grafted-polymer particles are crosslinked with a gelatin hardener selected from the following groups: bisvinylsulfonylmethane ether, bisvinylsulfonylmethane, carbamoylonium compounds, dication ether compounds, carbodiimide compounds. The element of claim 7 further comprising a dispersion of oxidized developer scavenger to prevent color contamination.

Inventors: BAGCHI PRANAB, CHEN TIENTEH, DANNHAUSER THOMAS JOSEPH, LEWIS JOHN DEREK, WHITSON MARK ANTHONY, BAGCHI, PRANAB, CHEN, TIENTEH, DANNHAUSER, THOMAS JOSEPH, LEWIS, JOHN DEREK, WHITSON, MARK ANTHONY, BAGCHI, PRANAB, C/O EASTMAN KODAK COMPANY, CHEN, TIENTEH, C/O EASTMAN KODAK COMPANY, Dannhauser, Thomas Joseph, Eastman Kodak Company, LEWIS, JOHN DEREK, C/O EASTMAN KODAK COMPANY, Whitson, Mark Anthony, c/o Eastman Kodak Co.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1994
Main classification: G03C1
IPC codes: G03C1, G03C7